The present invention relates generally to techniques for altering the resistivity of polysilicon circuit elements, and more specifically such techniques for conditioning polysilicon resistors for use in integrated circuitry.
Electronic circuit elements that are capable of at least temporarily changing logic states have heretofore been extensively used as memory elements in electronic circuitry. For example, such elements having two distinct and discernable logic states may be used to store binary information, as is known in the art.
One example of an electronic circuit element that may be used to store binary information is a so-called fusable link, or fuse. Fuses are typically formed in integrated circuitry as sheets or strips of electrically conductive material having as one logic state a conductive path therethrough and as an opposite logic state a fused open condition. Several types of conductive materials have been used for constructing such fuses in the past including, for example, circuit metalization, polysilicon, and the like, wherein a collection of such fuses are typically used to form one or more arrays or matrices of fuses (often referred to as memory blocks) for storing binary data. Such memory blocks may be programmed with binary data by selectively opening certain ones of the fuses while leaving others in their conductive state, as is known in the art.
Fuses of the foregoing type are typically referred to as nonvolatile memory elements in that the operational states thereof (i.e., either normally conductive or fused open) do not change when electrical power is removed from the memory unit. One popular material for forming such fuses is polysilicon. Polysilicon sheets or strips are fabricated in a conventional manner to form polysilicon resistors that fuse open at reasonably low current levels, operate reliably over wide temperature ranges, and are electrically, mechanically and chemically stable.
Although polysilicon resistors of the foregoing type have been successfully used in many integrated circuit applications to form memory elements as described, they have certain drawbacks associated therewith. For example, as with most integrated circuit resistors, polysilicon resistors often exhibit circuit-to-circuit and wafer-to-wafer resistance variations. Outlier resistors may accordingly require programming voltage/current levels that are more or less than expected, and programming such resistors may accordingly be more difficult to accomplish. As another example, the fusing operation may create resistor debris that could eventually result in unwanted circuit behavior at any time. As still another example, fusable resistors typically require removal of the integrated circuit passivation layer in the vicinity of the fusing sites. As a result, the underlying circuitry is susceptible to chemical or mechanical damage.
What is therefore needed is a technique for conditioning polysilicon resistors for use as fuse elements that avoids at least some of the drawbacks of prior art polysilicon fusable resistor techniques. Such a technique may be used to condition polysilicon resistors prior to fusing, or alternatively to replace the fusing technique with an improved programming technique that provides similar functional results while avoiding some of the drawbacks associated with the fusing process.
The foregoing shortcomings of the prior art are addressed by the present invention. In accordance with one aspect of the present invention, a method of electrically conditioning a polysilicon circuit element comprises the steps of providing a polysilicon circuit element, and injecting a first magnitude of current into the polysilicon circuit element, wherein the first magnitude of current causes an irreversible change in the resistance of the polysilicon circuit element from a first resistance value to a second lower resistance value.
In accordance with another aspect of the present invention, an apparatus for electrically conditioning a polysilicon circuit element comprises a polysilicon circuit element defining a first resistance value, and a current source responsive to a first control signal to supply a first magnitude of current to the polysilicon circuit element, wherein the first magnitude of current causes an irreversible change in the resistance of the polysilicon circuit element from the first resistance value to a second lower resistance value.
In accordance with a further aspect of the present invention, a memory circuit comprises a number of polysilicon circuit elements each defining a first resistance value, a current source producing a first current level, and means for coupling the current source to desired ones of the polysilicon circuit elements to thereby cause the first current level to flow therethrough, wherein the first current level causes irreversible changes in resistances of the desired ones of the polysilicon circuit elements from the first resistance value to a second lower resistance value, the number of polysilicon circuit elements thereby forming a memory circuit having a binary code programmed therein.
One object of the present invention is to provide a technique for electrically conditioning polysilicon circuit elements.
Another object of the present invention is to provide for such a technique electrically by injecting a constant current level into a polysilicon circuit element to thereby cause an irreversible change in the resistance thereof from a first resistance level to a second reduced resistance level.
Still another object of the present invention is to form a binary memory circuit comprised of a number of polysilicon circuit elements wherein some of the number of such elements exhibit the first resistance level and others are electrically conditioned to exhibit the second resistance level.
These and other objects of the present invention will become more apparent from the following description of the preferred embodiments.